Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
3GPP Long-term evolution (LTE) complements the success of High Speed Packet Access (HSPA) with higher peak data rates, lower latency and an enhanced broadband experience in high-demand areas. This is accomplished with the use of wider-spectrum bandwidths, Orthogonal Frequency-Division Multiple Access (OFDMA) and SC-FDMA (i.e., single carrier) air interfaces, and advanced antenna techniques. These techniques enable high spectral efficiency and an excellent user experience for a wide range of converged IP services. UMTS operators are rapidly adopting and offering IP services such as rich multimedia (e.g., video-on-demand, music download, video sharing), VoIP (Voice over IP), PTT (push to talk) and broadband access to laptops and PDAs. Operators offer these services through access networks such as HSPA, HSPA+ and LTE. In LTE as described in 3GPP TS 36.300 technical specification for EU-TRAN, one serving evolved base node (eNB) communicates via an uplink (UL) and downlink (DL) channel with user equipment (UE), thereby providing legacy interoperability by not depending upon dual mode communications.
Automatic Repeat-reQuest (ARQ) is an error control method for data transmission that uses acknowledgments and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame. A timeout is a reasonable point in time measured after the sender sends the data frame such that, if the sender does not receive an acknowledgment before the timeout, it usually re-transmits the frame until it receives an acknowledgment or exceeds a predefined number of re-transmissions.
A variation of ARQ is known as “Hybrid ARQ” (HARQ), which is generally considered to have better performance—particularly over wireless channels—at the cost of increased implementation complexity. In HARQ forward error correction (FEC) bits are also added to any existing Error Detection (ED) bits. As a result, HARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions.
For LTE, whenever an RLC (Radio Link Control) transmitter sends a polling request to a receiver, the receiver can respond with a Status Report, which contains presumably up-to-date RLC PDU (protocol data unit) information at the receiver. One problem may arise when, at the time that the receiver receives the polling request, there could still be some RLC PDUs being delivered by the HARQ layer and, hence, any Status Report formed by the receiver at that point in time may not take into account those PDUs. For instance the HARQ introduces out-of-sequence data due to processes finishing earlier than another. As a result, when an RLC receiver receives a poll and takes a snap shot of the RLC status, the status may not be accurate. Some RLC PDUs can still be in the process of being delivered and not accounted by the status report.
It has been suggested that the receiver should wait for a timer before forming the Status Report so as to let the HARQ processes finish before reporting, enabling the HARQ to account for those PDUs in transit. This requires either a simple timer or gap detection (i.e., a more sophisticated method) at the receiver. The problem with this approach is a fixed delay will be incurred before the receiver can send a Status Report—even if there is nothing being transmitted on the HARQ processes.